Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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lipoproteins, various enzymes, and products of

enzyme reactions could also be involved. At present,

little information is available on the physicochemical

changes and on the behavior of starch and minor

constituents during processing, cooling, and storage

of pasta. It is well known that semolina quality varies

widely among durum wheat varieties. Semolinas from

certain durum wheat varieties grown in some loca-

tions produce pasta with a good cooking quality and/

or yellow color, whereas other durum wheat varieties

give semolina that produces pasta with insufficient

cooking characteristics or pigment. Consequently, at-

tention is paid to check the quality of durum varieties

in order to produce the desired semolina.

0007 Common soft wheat flour is less pigmented, trans-

lucent, and flinty than durum wheat semolina, and

gives a gluten with an elastic strength suitable for

making bread, but not for manufacturing a superior

pasta product. However, flours milled from common

hard wheats with strong gluten, a high protein con-

tent, and superior rheological characteristics are ac-

ceptable for producing pasta. The color and cooking

quality of pasta from common wheat flour can be

improved by adding proportional amounts of egg. As

far as the water used for pasta making is concerned, it

should be pure drinking water with no off-flavors.

Method of Manufacture

0008 In a modern pasta factory, pasta products are formed

by extrusion on large automatic machines that

perform several processing operations. Production

lines for processing short and long pasta are shown

in Figure 1. The essential equipment includes:

0009A series of metal silos that receive the various types

of semolina or flour (coming either directly from a

mill adjoining the pasta factory or from outside

mills);

0010An automatic continuous press;

0011A spreader predrier for long pasta or shaker pre-

drier for short pasta;

0012A drier; and

0013A storage and packing unit.

(See Extrusion Cooking: Principles and Practice.)

Mixing

0014The automatic press is one of the most important

machines in a pasta factory. It performs three essen-

tial processing operations: mixing, kneading, and

extruding.

0015In the mixing stage, water is added to the semolina

so that the moisture content of the dough is approxi-

mately 30%. The flow of ingredients is regulated

by volumetric or gravimetric dosers with constant

and proportioned outputs. It is very important that

the water is absorbed evenly by the semolina or

flour particles to obtain a thoroughly homogeneous

dough that will prevent the dried pasta from having

faults (white spots, for instance). For this reason, the

Press

(a) Shaking predrier Drier

(b) Spreader predrier

Press

Drier Cooler

Cooler

fig0001 Figure 1 Modern production lines for processing (a) short and (b) long pasta. Reproduced from Pasta and Macaroni, Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

PASTA AND MACARONI/Methods of Manufacture 4375

semolina particles must have the same size (for a

typical semolina, a maximum of 25% of particles

should pass through a 0.180-mm mesh sieve), and

the time needed for the liquid to be absorbed by the

particles has to be evaluated while taking into ac-

count their average size and the temperature of both

flour and liquid. Steps should be taken to insure that

the lower the flour temperature, the higher the tem-

perature of the liquid in the dough.

0016 Depending on the pasta shape and type, warm

water at 40–65



C is generally used. A uniform

dough is obtained by mixing the ingredients in a

special twin-shaft mixing chamber for 15–17 min

for short pasta, and 16–20 and 16–18 min for long

pasta and long egg pasta, respectively. The shafts of

the mixer turn in opposite directions to limit the

amount of dough balling that can occur.

0017 The latest development in press technology is rep-

resented by new systems that are able to carry out the

dough-making in a very short time (about 2–3 min)

instead of the 15–20 min necessary for normal dough

preparation. A very short dough-making time in-

creases considerably the line production speed, since

it can better match the speed of the following steps

(predrying, drying, etc.). For this reason, at present,

fine granulometry semolina is preferred, which best

suits short dough-making times. Moreover, short

dough-making times allow for a reduction in the

press size (output capacity being equal) and conse-

quently a better plant compactness and smaller

dimensions.

0018 Modern continuous presses are equipped with

vacuum-producing equipment to remove any air

bubbles from the pasta dough during mixing and

just before extrusion. The aim of this operation is to

avoid the formation of small bubbles, which can give

the end product a white, chalky appearance. In add-

ition, air bubbles can facilitate the oxidation of pig-

ments, resulting in a pale and unattractive pasta, and

can weaken the dried product.

Extruding

0019 The extrusion screw is the core of the pasta press, and

special attention is paid to its design and construc-

tion, just as to the compression cylinder. In fact, it is

vital to prevent the dough from being overworked, an

occurrence that would adversely affect the quality of

the finished product. The dough is forced by the

screw into the extrusion head. Even under the best

operative conditions, a considerable amount of heat

is generated during this process; therefore, the barrel

and the extrusion head are equipped with a water-

cooling jacket to reduce the heat and to maintain a

constant extrusion temperature of no more than



C. Higher temperatures could cause the deterior-

ation of the cooking quality of the finished product.

0020The process in the press is completed by the extru-

sion of the dough through a die to make a variety of

product shapes that are cut to the required length

with a rotary cutter. Until recently, pasta dies were

made of bronze. Now, new dies with Teflon on the

extruding surface are used. Teflon extends the life of

the die and improves the appearance of the product.

Bronze dies are still used for small special productions

characterized by the rough surface of pasta. Special

equipment is required to form particular shapes such

as nests, skeins and Bologna-type pasta. The cut pasta

is then hung over drying rods or spread evenly, all

automatically, ready for the drying stage.

Drying

0021Drying is the most difficult and critical step in the

pasta-making process. The objective of this process is

to decrease the moisture level of the extruded pasta

from about 30% to less than 12.5% without causing

any damage to the finished product. To achieve this,

the air temperature and relative humidity must be

properly controlled. If the drying is too slow, the

pasta products tend to spoil or become moldy. How-

ever, if they are dried too rapidly, moisture gradients

occur that may cause checking or cracking. Usually,

uniform temperature and ventilation are insured in all

parts of the dryer. This is achieved through a suitable

hot-air circulation system using low-pressure spiral

ventilators located in various parts of the dryer.

Moreover, automatically controlled periods of hot-

air circulation alternate with periods of rest to allow

for moisture equilibration between the inner part of

the pasta and its surface.

0022In modern plants, the drying cycle can be sub-

divided into three distinct stages: (1) predrying, (2)

drying, and (3) cooling. Although identical basic

components are used, the manufacture of different

types of pasta requires machines that are technically

different. The most widely used dryer for long pasta is

basically made of a single complex consisting of:

0023an extruded-pasta supporting structure on one or

more levels;

0024a separate drive unit for the section on one or more

levels;

0025a heat ventilation unit;

0026an unloading unit; and

0027a regulation and control unit.

The initial predrying takes place in the first part of the

dryer, where special aluminum or steel rods loaded

with pasta are normally at only one level. During this

stage, the product moisture is reduced to 17–18%,

4376 PASTA AND MACARONI/Methods of Manufacture

as a consequence of quick evaporation of the inside

water excess of pasta. Surface drying has to be

avoided to prevent closing of the interstices through

which the remaining water can reach the surface by

capillarity and evaporates. This predrying stage lasts

a short time and is followed immediately by a longer

drying stage.

0028 For the final drying, driers with more than one level

are used. The product hanging on the rods is moved

by means of special conveyor chains in the one-level

section and with a system of racks, which alternate

vertical and horizontal movements, in the section

with more than one level. From a technological

point of view, the drying differs from the predrying

phase for a more gradual moisture reduction and

particularly for the alternation of water evaporation

and its even distribution inside the product.

0029 The predrying stage of short pasta is generally

carried out with vibrating trays, called shaker pre-

dryers, whereas there is a great diversification in the

machines used for the next drying steps. Two differ-

ent kinds of equipment are used: the belt dryer and

the rotary dryer. The belt dryer is more suitable for

large shapes, which must be treated delicately. These

dryers receive the product from the shaker predriers

by means of conveyor belts. An oscillating band

evenly distributes the product on the whole upper

belt width. The product, after having reached the far

end of the belt, falls on the following ones, repeating

this route as many times as the existing number of belt

dryer tiers. Drying is achieved by means of aerother-

mal units in which hot water is kept in forced circula-

tion by a pump. The rotary dryers can be used for

both the preliminary and final drying of small- and

medium-sized and Bologna-style pasta products. The

rotary driers are essentially composed of a rotating

drum, supported on rollers and driven by a multiple-

speed gearbox. The product advances forward one

cell for each revolution of the drum. Drying is

achieved by means of a battery of air-circulating

fans and heat radiators that are located above the

drum.

0030 At present, the drying technology varies greatly

from factory to factory and from country to country,

the main difference being in the degree of temperature

used during the process. However, two systems can be

recognized: (1) traditional or conventional drying,

which uses ‘low’ temperatures not higher than



C, and (2) ‘high’-temperature drying, which uses

temperatures between 70 and 85



C, even though

there is a trend to seek higher temperatures. Of

course, the total drying time varies, depending

on the system employed. High-temperature drying

has been without doubt the most important innov-

ation in pasta technology in recent times. This new

technique has the advantage of appreciably reducing

the drying time, compared with traditional methods,

and of reducing microbial contamination. The former

characteristic allows the construction of drying

lines that maintain the same production capacity

as the traditional lines, but they are smaller, more

compact and constructed to facilitate maintenance

operations.

0031With the increasing popularity of high-temperature

drying among pasta manufacturers, considerable

attention has been given to the effect of this treat-

ment on the cooking quality, color, and chemical

composition of the final product. The effect of high-

temperature drying on the cooking quality is still

controversial. The main reason is that the available

information comes from investigations where the

method of application of the high temperature during

the drying cycle and the intrinsic characteristics of the

raw materials are different. However, experimental

evidence is accumulating to prove that semolina

protein quality and quantity can influence cooking

quality jointly or independently as a consequence of

the drying technology adopted (i.e., low- or high-

temperature drying). Different temperatures modify

components and influence cooking quality in differ-

ent ways. At low temperatures (40–50



C), protein

quantity and quality are equally important in deter-

mining the final pasta results, whereas at high

temperatures (80



C), protein quantity is more im-

portant and is strongly correlated with the resultant

pasta quality. Some researchers have reported that

pasta proteins are in fact polymerized by high drying

temperatures, whereas starch granules appear embed-

ded in the protein matrix. As a consequence, there

may be an improvement in the cooking quality of

pasta. Pasta cooking quality is determined by a phys-

ical competition between protein coagulation into a

continuous network and starch swelling with spher-

ules scattering during cooking. If the former prevails,

starch particles are trapped in the network alveoli,

promoting firmness in cooked pasta, whereas if the

latter prevails, the protein coagulates in discrete

masses lacking a continuous framework, and pasta

will show softness and usually stickiness on cooking.

High-temperature drying partially overcomes this

competition by producing a coagulated protein

framework in dry pasta without starch swelling.

0032Drying conditions also have a significant effect on

the retention of lysine in spaghetti. As lysine is the

limiting amino acid in wheat, any reduction in its

availability has an important bearing on the nutri-

tional quality of pasta. A decrease in the amount of

total lysine has been reported as being directly pro-

portional to the increase in temperature. The loss of

lysine depends mainly on the blockage of the amino

PASTA AND MACARONI/Methods of Manufacture 4377

group as a consequence of the Maillard reaction. (See

Browning: Nonenzymatic.)

Cooling and Packaging

0033 The final drying is followed by a cooling treatment

that must be carried out with certain precautions to

avoid damage to the pasta through humidity imbal-

ance within the shape. After this stage, the end

product is unloaded, sent to storage silos, and then

sent to the packaging section.

0034 Pasta products are packaged in various ways,

depending on their particular shapes and sizes. The

most widely used packages are cellophane bags and

packets, cartons, and special trays. All these are

automatically weighed, packed, and collected by

machines on continuous cycles. Sometimes, semi-

automatic machines may be required.

Considerations of Quality

0035 A good cooking quality is the most important re-

quirement for pasta products. Aroma, taste, and,

sometimes, uniformity of shape and color and break-

ing strength of uncooked pasta are also important to

the consumer. Although it can be interpreted

according to the individual taste and habits of the

consumer, the cooking quality of pasta is generally

regarded as the capacity of the product to maintain its

shape when cooked in boiling water and to retain a

good texture after cooking without becoming a thick,

sticky mass. The parameters involved in the evalu-

ation of pasta cooking quality can be defined as

follows:

0036 stickiness is the state of surface disintegration of

cooked pasta;

0037 firmness is the resistance of cooked pasta when

sheared between the teeth;

0038 bulkiness is the degree of adhesion of pasta shapes

after cooking.

As already mentioned, cooking quality depends

essentially on the type of raw materials used and on

their intrinsic characteristics, although it can be

affected by the manufacturing conditions.

0039 As far as the nutritional quality of pasta is con-

cerned, the nutritive value appears to be completely

independent of the use of durum wheat semolina or

bread wheat flour. However, the addition of other

ingredients such as eggs, vegetables, soy or vegetable

protein concentrates and isolates, individual nutri-

ents, etc. produces remarkable changes in the

nutritional value of common pasta.

0040 The protein quality of pasta can be influenced by a

reduced bioavailability of lysine caused by severe

drying conditions. This has to be considered when

adding high-quality protein integrators to the raw

materials.

See also: Browning: Nonenzymatic; Extrusion Cooking:

Principles and Practice; Wheat: The Crop; Grain

Structure of Wheat and Wheat-based Products

Further Reading

Cubadda R (1988) Evaluation of durum wheat, semolina,

and pasta in Europe. In: Durum Wheat: Chemistry and

Technology, pp. 217–228. St. Paul, MN: American

Association of Cereal Chemists.

Cubadda R (1989) Current research and future needs in

durum wheat chemistry and technology. Cereal Foods

World 34: 206–209.

Cubadda R (1996) Pasta quality: the relationship between

raw material properties and production technologies.

Proceedings of 1st World Pasta Congress, pp. 164–

168. Pinerolo, Italy: Chiriotti Editori.

D’Egidio MG, Mariani BM, Nardi S, Novaro P and

Cubadda R (1990) Chemical and technological variables

and their relationships: a predictive equation for pasta

cooking quality. Cereal Chemistry 67: 275–291.

Grzybowski RA and Donelly BJ (1979) Cooking properties

of spaghetti: factors affecting cooking quality. Journal of

Agricultural and Food Chemistry 27: 380–384.

Lirici L (1999) From the bar kneader to the modern auto-

matic press: over 100 years of evolution. Tecnica Moli-

toria 50: 101–120.

Pavan G (1996) Pasta: processing trends and possible tech-

nological answers. Proceedings of 1st World Pasta Con-

gress, pp. 79–94. Pinerolo, Italy: Chiriotti Editori.

Resmini PP and Pagani MA (1983) Ultrastructure studies of

pasta. Food Microstructure 2: 1–12.

Dietary Importance

L J Malcolmson, Canadian International Grains

Institute, Winnipeg, Manitoba, Canada

Background

0001The term ‘pasta’ is used to describe products made

from a flour–water dough that fit the ‘Italian’ style of

extruded foods. The primary ingredient used in the

manufacture of pasta is a coarsely ground flour from

durum wheat, called semolina. In the commercial

manufacture of pasta, semolina and water are mixed

into a dough, which is then extruded through a die to

produce the desired size and shape of pasta. The prod-

uct is then dried or sold fresh. Common pasta products

4378 PASTA AND MACARONI/Dietary Importance

include spaghetti, vermicelli, linguine, rotini, rigatoni,

and macaroni.

0002 In contrast, the term ‘noodle’ is used to describe an

‘Oriental’ style of sheeted and cut products made

from grains other than durum wheat. Grains used to

produce noodles include common or bread wheat,

rice, barley, and buckwheat, as well as legumes such

as mung bean.

0003 The earliest written record of pasta in the Italian

diet was reported to be 1279, although noodle prod-

ucts were most certainly part of earlier cultures of

China and the later cultures of many other countries.

Today, pasta products have become an important part

of the diet in many countries. This has been influ-

enced in part by changes in consumers’ perception of

pasta. Once thought of as starchy and fattening, pasta

is now considered to be nutritious and convenient.

Table 1 lists the average per capita consumption of

uncooked pasta in selected countries. The low con-

sumption of pasta products in Asian countries such as

China and Japan compared with other countries is a

reflection of the much higher consumption of noodle

products in these countries compared with ‘Italian’-

style pasta products.

0004 The widespread consumption of pasta throughout

the world is likely due to its simple formulation,

relative ease of processing, extended shelf-life in the

dry form, low cost, and versatility. Pasta’s versatility

stems from the fact that it is available in numerous

shapes and sizes, and can be prepared and served with

other foods as an appetizer, main dish, side dish,

salad, soup, or dessert, thus allowing menu flexibility

for the consumer. Because pasta is relatively bland in

flavor, it lends itself to be complemented by sauces

and other food ingredients.

Nutritional Value of Pasta

0005Although the nutritional content of pasta can vary

widely, depending on the ingredients used in its prep-

aration and added sauces, pasta provides significant

quantities of complex carbohydrates, protein, B vita-

mins, and iron, and is low in fat. A typical 140-g

serving of cooked macaroni or spaghetti provides

approximately 6 g of protein, 40 g of carbohydrates,

and 2 g of dietary fiber, whereas whole wheat pasta

provides approximately 8 g of protein, 38 g of carbo-

hydrates, and 3 g of total dietary fiber. The fat content

of a 140-g serving of pasta is less than 1 g.

0006Fortification of pasta products with thiamin, ribo-

flavin, niacin, iron, and, more recently, folic acid is

permitted in many countries. The vitamin and

mineral content of enriched and whole wheat pasta

is provided in Table 2.

0007In recent years, studies have been undertaken to

examine the beneficial role of pasta in the diet. Evi-

dence exists indicating that a diet rich in pasta may

lower the incidence of cardiovascular disease, be

helpful in managing diabetes, and be useful to

individuals with a specific gluten intolerance or

individuals suffering from celiac disease.

0008According to a review published by Costantini,

subjects who consumed diets rich in pasta had lower

levels of serum low-density lipoproteins and total

lipoproteins than those who did not consume pasta-

rich diets. This finding suggests that diets rich in pasta

may be responsible for lower incidents of cardio-

vascular disease, provided the pasta is prepared and

served using other low-fat ingredients.

0009The exact role of pasta in the dietary management

of diabetes is not fully understood. However, it has

been speculated that diets rich in pasta result in

reduced rates of both starch uptake and gastric

emptying, thereby resulting in a lowered postprandial

blood glucose and insulin response.

tbl0001 Table 1 Average yearly per capita consumption in selected

countries

Country Percapita consumption (kg)

Italy 28.0

Venezuela 12.7

Switzerland 9.1

USA 9.0

Russia 7.0

France 7.0

Greece 6.6

Canada 6.3

Germany 5.0

Spain 4.0

UK 2.0

Japan 1.8

China 0.8

Source: Pastavilla Pasta (2001) and Pastificio Pagani (2001).

tbl0002 Table 2 Vitamin and mineral content of cooked pasta (140-g serving)

Product Thiamin (mg) Riboflavin (mg) Niacin (mg) Folate(mg) Iron (mg) Calcium (mg) K (mg) P (mg) Mg (mg) Zinc (mg)

Enriched 0.20 0.20 2.4 10.0 2.0 10.0 44 76 26 0.8

Whole wheat 0.20 0.00 1.0 8.0 1.4 22.0 62 124 42 1.2

K, potassium; P, phosphorus; Mg, magnesium. Source: NutriQuest

(1999).

PASTA AND MACARONI/Dietary Importance 4379

0010 Peptides derived from the gliadins of durum wheat

have been shown to be less aggressive to celiac dis-

ease. Thus, pasta derived from durum wheat may be

useful in the diets of individuals suffering from gluten

intolerance, depending on the severity of the reaction.

0011 The nutritional value of pasta can be increased by

the addition of other food ingredients to increase the

protein or fiber content. Research has been under-

taken to investigate the effects of various protein

sources on pasta quality. Studies have been published

showing the successful addition of whey derivatives,

nonfat dry milk, soy, corn, legumes (bean, lentil,

cowpea), lupin, amaranth, buckwheat, and maize.

By blending durum wheat semolina with other plant

protein sources, the limiting amino acids lysine and

threonine are compensated for by the other

protein sources. Protein-enriched pasta products are

currently available in some countries. Egg-enriched

pasta is also popular in many countries. Whole grain

cereals such as barley and legumes as well as the use

of whole wheat have also been used to increase the

mineral, vitamin, and dietary fiber content of pasta.

0012 The addition of natural colorants such as black

squid ink, pure

ed spinach and carrot, tomato paste,

and beet juice has also received considerable atten-

tion, and many of these products are currently being

sold in the marketplace. The use of herbs and choc-

olate has also gained popularity in the production of

more flavorful product lines. These ingredient add-

itions are made to enhance the flavor and color of the

final product, since the nutritional properties are not

changed substantially.

Effects of Processing and Cooking on the

Nutrient Content of Pasta

0013 The effect of drying and cooking on the nutritional

content of pasta has also been studied. Drying condi-

tions have been found to significantly affect the bio-

availability of lysine retention in spaghetti. Losses as

high as 47% have been found for spaghetti dried at



C and as low as 22% for pasta dried at 45



Other researchers have reported that the extent of

lysine losses is a function of both drying temperature

and length of exposure to high drying temperatures.

With continued improvements in drying technologies,

the deleterious effects of high-temperature drying on

available lysine will likely be minimized. Further-

more, whether these losses are significant, given that

pasta is combined with other foods in the diet, is also

questionable. Studies that have examined the loss of

vitamins during drying reported higher losses of thia-

min and riboflavin with increasing drying tempera-

tures. These losses were smaller for thiamin under

industrial drying conditions. The effect of cooking

on the retention of B vitamins has suggested that

a good portion, namely 50–75% of the vitamins, is

retained following cooking. With fortification of B

vitamins during processing, these losses are not as

significant as they may be in products that are not

fortified.

0014As a wheat-derived staple food, pasta is second

only to bread in world consumption. Its worldwide

acceptance can be attributed to its low cost, ease

of preparation, versatility, long shelf-life, and nutri-

tional and sensory properties. With the greater em-

phasis towards the consumption of cereal based foods

as a result of changes in dietary nutritional recom-

mendations, the importance of pasta in the diet of

many countries will continue to remain high.

See also: Celiac (Coeliac) Disease; Coronary Heart

Disease: Etiology and Risk Factor; Pasta and Macaroni:

Methods of Manufacture; Wheat: Grain Structure of

Wheat and Wheat-based Products

Further Reading

Costantini AM (1986) Nutritional and health significance

of pasta in modern dietary patterns. In: Mercier Ch and

Cantarelli C (eds) Pasta and Extrusion Cooked Foods:

Some Technological and Nutritional Aspects, Techno-

alimenti Food Technology and Nutrition Series, No. 1.

London: Elsevier Applied Science.

Cubadda R (1986) Effect of the drying process on the

nutritional and organoleptic characteristics of pasta. A

review. In: Mercier Ch and Cantarelli C (eds) Pasta and

Extrusion Cooked Foods: Some Technological and

Nutritional Aspects, Technoalimenti Food Technology

and Nutrition Series, No. 1. London: Elsevier Applied

Science.

Dick JW and Matsuo RR (1988) Durum wheat and pasta

products. In: Pomeranz Y (ed.) Wheat: Science and Tech-

nology, vol. II. St. Paul, MN: American Association of

Cereal Chemists.

Giese J (1992) Pasta: New twists on an old product. Food

Technology 46(2): 118–126.

Ranhotra GS, Gelroth JA, Novak FA and Mathew RH

(1985) Retention of selected B vitamins in cooked

pasta products. Cereal Chemistry 62(6): 476–477.

Seibel W (1996) Future trends in pasta products. In: Kruger

JE, Matsuo RB and Dick JW (eds) Pasta and Noodle

Technology. St. Paul, MN: American Association of

Cereal Chemists.

4380 PASTA AND MACARONI/Dietary Importance

Pasta Filata Cheese See Cheeses: Types of Cheese; Starter Cultures Employed in Cheese-making;

Chemistry and Microbiology of Maturation; Manufacture of Extra-hard Cheeses; Manufacture of Hard and Semi-

hard Varieties of Cheese; Cheeses with ‘Eyes’; Soft and Special Varieties; White Brined Varieties; Quarg and

Fromage Frais; Processed Cheese; Dietary Importance; Mold-ripened Cheeses: Stilton and Related Varieties;

Surface Mold-ripened Cheese Varieties; Conjugated Linoleic Acid

PASTEURIZATION

Contents

Principles

Pasteurization of Liquid Products

Pasteurization of Viscous and Particulate Products

Other Pasteurization Processes

Principles

R A Wilbey, The University of Reading, Reading, UK

Introduction

0001 Pasteurization has become widely accepted as an

effective means of destroying vegetative pathogens

in food products, with the least possible damage to

the sensory qualities of the product. The heat treat-

ment also reduces the general microbial population,

so that an increased shelf-life is normally obtained.

Bacterial spores and some heat-resistant enzymes will

survive pasteurization processes and limit the shelf-

life of the product.

Historical Origins

0002 Cooking is an age-old method of preparing many

traditional foodstuffs, and for centuries it was gener-

ally appreciated that cooked products would nor-

mally take longer to putrefy than if they were raw.

0003 In the latter part of the 18th century there was great

interest in understanding the mechanism of putrefac-

tion. Lazzaro Spallanzani demonstrated that putre-

faction may not occur in a heated sealed flask of an

infusion, but that aerial contamination could result in

putrefaction. The presence of microorganisms was

demonstrated, and it was recognized that there was

a possible division into organisms that could be killed

by boiling and those that would survive this heat

treatment. Subsequent experiments by Franz Schutz

in the early 19th century demonstrated that it was not

the air itself that caused spontaneous putrefaction,

but a contaminant carried in the air. Similar experi-

ments were carried out by Theodore Schwann at the

same time.

0004Methods for the preservation of foodstuffs were

developed in parallel with this pioneering work on

the basic understanding of why foods spoil. Carl

Wilhelm Scheele used heat for the conservation of

vinegar, and Nicholas Appert developed the preserva-

tion of foods by heating in cans. In 1824 William

Dewees recommended that milk for infants be heated

to near to boiling (but not boiled) then cooled as

preparation for infant feeding.

0005The credit for the development of a mild method of

processing foods, now particularly associated with

milk, has been given to Louis Pasteur, after whom

the pasteurization process was named. Pasteur had,

amongst his many interests, an interest in fermenta-

tions. The poor hygiene conditions associated with

the production of food and beverages at that time

often led to unwanted fermentations, causing putre-

faction and loss of product. His experiments con-

firmed that fermentations were not spontaneous but

the result of microbial metabolism. While some of his

earlier work was with lactic fermentations, most of

his work in this field was based on alcoholic fermenta-

tions, brought about by yeasts. The conversion of

ethanol to acetic acid was demonstrated to be brought

about by bacteria, subsequently classified as Aceto-

bacter. Both yeasts and Acetobacter could be destroyed

by relatively mild heat treatments, at about 55



Cinan

acid medium such as wine, in closed vessels.

PASTEURIZATION/Principles 4381

0006 While Pasteur’s work on beer, wine, and vinegar

laid the foundations for hygienic processing, his com-

plementary work on the relation between specific

organisms and disease also aided the recognition of

the public health implications of hygiene and of heat

treatments.

0007 By the late 19th century the economic benefits from

improving the shelf-life of milk and other products

were appreciated, though the microbiological and

public health implications of pasteurization were

not fully understood. Pasteurization of wine was

adopted in both France and the USA and is still used

for some wines, though filtration, higher alcohol

levels, and better production hygiene have largely

displaced heat treatment.

Introduction to the Dairy Industry

0008 The heat treatment of milk on a commercial scale

did not develop until the end of the 19th century,

with the production of commercial pasteurizers

in Germany and Denmark. The earlier treatment

systems were aimed at improving the storage life of

milk, often using simple continuous-flow techniques

to reduce costs. The realization that milk was a po-

tential carrier for diseases such as tuberculosis led to

the development of a low-temperature long-time

(LTLT) batch process, the first commercial plant

being installed by Charles North in New York in

1907. It was not till 1922 that pasteurization was

legally recognized in the UK when the term was de-

fined in the Milk and Dairies (Amendment) Act, using

a LTLT process at 62.8–65.5



C for a minimum of

30 min.

0009 LTLT pasteurization was the first safe method

adopted, but the processing of milk was revolution-

ized by the invention in the UK of the plate heat

exchanger, capable of regeneration. The development

of a modular heat exchanger that could be relatively

easily cleaned, together with a microbiologically ef-

fective holding tube system and a flow diversion valve,

enabled milk to be heat-treated with safety on a far

larger scale than had been possible with the batch-

based LTLT system. With better appreciation of the

thermal death characteristics of pathogens, this con-

tinuous process was able to take advantage of higher

process temperatures with a corresponding shorter

hold, and became known as the high-temperature

short-time (HTST) process. In the UK, as in the Euro-

pean Community, the minimum heat treatment

permitted is 71.7



C for 15 s. It has been suggested

that 15 s was originally chosen as the minimum time

to allow an adequate margin for the response rate of

the temperature-sensing and control system at that

time.

0010Subsequent developments in the design of process

equipment have led to the construction of pasteuriza-

tion plants that may be cleaned in place (CIP), with

much greater thermal efficiency and with much more

sensitive and responsive instrumentation and control

systems. It is now technically possible to pasteurize at

higher temperatures with little or no hold – the so-

called ‘flash’ processes.

0011Development of safe HTST processes for milk

insured that in-bottle LTLT pasteurization of milk,

pioneered by Charles North in 1911, although

capable of producing a high-quality product virtually

free of postpasteurization contamination, was too

expensive and thus unable to compete in the market-

place. The only in-bottle milk processing that con-

tinued was for sterilized milk and this product is also

diminishing with the development of continuous

UHT processes. (See Heat Treatment: Ultra-high

Temperature (UHT) Treatments.)

0012The LTLT process has found an application in

cream processing. With most UK dairies now produc-

ing over half of their milk as semiskimmed plus some

skimmed milk, large but variable volumes of cream

are produced from the separation and inline stand-

ardization processes. Carrying out separation and

cream holding at 63



C avoids problems of varying

throughputs with HTST processes.

0014The drive to minimize energy costs has led to the

widespread use of regeneration in plate heat exchan-

gers, where the cold raw material is preheated by the

hot pasteurized product. This process can enable heat

recoveries in excess of 90%, giving energy savings for

both heating and refrigeration. There is a risk associ-

ated with this process, however, as in simple systems

with one pump, the pressure of the raw liquid will be

higher than that of the pasteurized product, so if there

were a leak within the heat exchanger there could be

contamination of the pasteurized product. Apart

from regular checks on the integrity of the heat

exchanger, for many products the problem can be

avoided by introducing a second pump after the pre-

heater plus some means of maintaining a higher pres-

sure in the downstream section of the heat exchanger,

either by flow restriction within the heat exchanger or

by a backpressure valve at the exit. Where such pres-

sure drops are undesirable, double heat exchanger

plates may be used. These pairs of thinner heat

exchanger plates are constructed without gaskets

between the pairs so that, in the event of one of the

pair failing, the leaking liquid would leak out through

the gap between the plates to give a failure indication

without the risk of contamination of the other flow

path. Maintenance of heat exchanger plates has been

eased by the introduction of clip-in gaskets that do

not require adhesive.

4382 PASTEURIZATION/Principles

Basic Aims of Process

0015 The basic aim of pasteurization is summarized by

the definition adopted by the International Dairy

Federation (IDF): ‘pasteurization is a process applied

to a product with the aim of avoiding public health

hazards arising from pathogenic microorganisms

associated with milk by heat treatment which is

consistent with minimal chemical, physical and

organoleptic changes in the product’.

0016 This definition would be equally applicable to

other commodities if one were to substitute the

name of that product for ‘milk’ in the definition.

0017 To achieve the public health objective of pasteur-

ization in a particular product, it is essential to be

aware of the pathogens associated, or potentially

associated, with that product. The thermal death

characteristics of the organism(s) in that product

must also be known.

0018 In most of the early work the death characteristics

were expressed in terms of a temperature–time com-

bination that would destroy the target pathogen. In

the case of milk, tuberculosis was recognized as a

major disease associated with milk consumption and

Mycobacterium tuberculosis was found to be the

most heat-resistant pathogen normally associated

with milk. Temperature–time combinations needed

to destroy M. tuberculosis were published by North

in 1911, North and Park in 1927, Hammer in 1928

and Dahlberg in 1932; these data are included in

Figure 1.

0019 This knowledge enabled safer process conditions to

be set up for LTLT and subsequently for HTST pro-

cesses. However, with better understanding of the

kinetics of thermal death rates, more quantitative

data may now be obtained.

0020 When organisms are subjected to a moist heat

above their normal temperature range there is a

drop in the number of survivors with time: the loga-

rithm of the number of survivors being inversely

proportional to the exposure time. Thus for a given

temperature, the time taken for a 10-fold reduction in

survivors, the D value, may be obtained. D values are

expressed in minutes or seconds and must be accom-

panied by the temperature, e.g., D

for 72



C. While

D values are a good approximation for the behavior

of bacterial populations, there can be a ‘tail’ of more

heat-resistant strains leading to an overestimate of the

lethality of the process. The term D may also be used

without a subscript to describe the number of orders

that the population may be reduced by in the course

of a thermal or other process, e.g. a 10

reduction

may be described as a 12D process.

0021 The D value will decrease with increasing tempera-

ture. The rate of change is usually given as a z value,

the z value being the change in temperature required

to give a 10-fold change in the D value. Typical z

values for mesophiles are 4–6



C in high water activ-

ity (a

) systems, though some authorities use a value

of 8



C. The data in Figure 1 for M. tuberculosis

implies a z value of 6.3



C. Some care is needed in

using z values as they can vary with temperature and,

as with the D values, will vary with the product being

heated. Capillary tubes have been used for the esti-

mation of thermal death data. While this method may

be appropriate for low-viscosity foods, increases in

viscosity during the heat treatment may give different

results to those from pilot and full-scale treatments.

0022By using the D and z values it is possible to estimate

the risks associated with a temperature–time combin-

ation, i.e., the probable level of survivors for a given

level of contamination in the raw material. This is

easy for a batch process such as with LTLT pasteur-

ization, since the hold is easily measured and the

contribution of the heating and cooling stages to the

overall lethality of the process is small. With HTST

processes, however, the temperatures are higher and

the heating and cooling stages may have a significant

contribution to the overall lethality of the process.

Thus it is essential that the process be character-

ized in terms of temperature and minimum time.

10 000

1000

100

60 65 70

Temperature (˚C)

Time (s)

fig0001Figure 1 Time–temperature conditions for destruction of

Mycobacterium tuberculosis in milk. Continuous line, based on

data from North and Park (squares), Hammer (triangles), and

Dahlberg (circles). Dashed line, virtual loss of alkaline phosphat-

ase. Dotted line, reduction in cream line. Adapted from North and

Park (1927) The American Journal of Hygiene 147: VII March,

cited by Davis JG (1955) A Dictionary of Dairying p. 787. London:

Leonard Hill.

PASTEURIZATION/Principles 4383

Minimum time is critical as the microbiological risk

(particularly the public health risk) is associated with

the minimum heat treatment given to the product.

Since most HTST processes are continuous, the flow

characteristics of the system must be taken into ac-

count. From a microbiological viewpoint, turbulent

flow in the pasteurizer will give the best results as

there will be a narrower spread of flow rates and

hence residence times in the equipment. However, in

practice, slower flow rates may be needed to conserve

desirable product characteristics or to avoid excessive

pressures.

0023 Once the plant has been characterized it is possible

to analyze the process quantitatively in relation to a

given risk. For instance, safe conditions for milk have

been established as a minimum hold of 15 s at 72



If the lethality of this process is given an arbitrary

value of one unit, for instance P*, which uses a z value

of 8



C, then we can calculate the contribution of each

stage of the process to the overall lethality. An alter-

native unit, the pasteurization unit (PU) has also been

used; in this case, taking 1 min at 60



C(z ¼ 10



C) as

the standard unit so that a safe process for milk at

that temperature would need to be 60 PU if 63



30 min is taken as the standard for a safe process. A

comparison between PU and P* values is made in

Figure 2. A temperature of 10



C is appropriate for

spores but is higher than normal for vegetative patho-

gens – appropriate z values should be chosen to meet

the substrate pathogen conditions applying to that

foodstuff. Changes in the pH or a

of the foodstuff

will alter the D and z values.

0024 It will be seen from Figure 2 that the contribution

of temperatures below 65



C to overall lethality in an

HTST process is so small that it may be ignored.

However, at higher temperatures the effect of heating

and cooling becomes more important. This may be

illustrated by comparing two model heat treatments

in heat exchangers where the rate of change of

temperature is 1



1

(Figure 3).

0025 With the HTST process A, the hold at 72



C pro-

vides the main contribution to the lethality of the

process; the heating and cooling stages contribute

less than 30% to the total lethality of the process.

Raising the final temperature to 80



C as in B extends

both the heating and cooling times and introduces

exposure to higher temperatures; the result is that,

despite there being no hold, the total heat treatment

is well in excess of the minimum safe treatment.

0026 While the primary concern in pasteurization is to

obtain a safe food, this is irrelevant if the sensory

quality of the food is reduced excessively, either by

overcooking or due to the persistence of other less

temperature-labile factors (microbiological or bio-

chemical). Cooked flavors may be acceptable in some

foods, e.g., clotted cream, but not in others such as

wine.

0027Heat treatments may also bring about undesirable

changes in the stability or functional properties of

food products, often related to protein denaturation.

In milk the denaturation of agglutinin (an immuno-

globulin fraction) reduces the rate at which cream

forms in milk on standing: the denaturation is more

pronounced when more severe heat treatments are

used so that less cream separates from the milk. This

has been an important consideration in processing

milk for bottling (Figure 1), where the consumer has

associated cream separation with milk quality, but it

is not relevant to the production of homogenized

milks where cream separation would indicate a pro-

cessing failure. Similarly, the protein denaturation

associated with pasteurization of liquid egg white

0.1

0.01

65 70 75

Temperature (°C)

Lethality

0.1

0.01

65 70 75

Temperature (°C)

P *

(a)

(b)

fig0002Figure 2 Lethal effect of a 1-s exposure at typical pasteuriza-

tion temperatures: (a) lethality; (b) P*. Straight line, P*(z ¼ 8



C);

dashed line, PU (z ¼ 10



C).

4384 PASTEURIZATION/Principles